The south of Java earthquake of 1921 September 11: A negative search for a large interplate thrust event at the Java Trench

We present a detailed study of the earthquake of 1921 September 11, located between the two anomalous 'tsunami earthquakes' of 1994 and 2006 south of Java. Based on modern relocation techniques, a compilation of focal mechanism constraints from historical seismograms, a quantification of mantle surface waves, and a numerical simulation of its mediocre tsunami (only 10cm at Cilacap), we conclude that the 1921 earthquake occurred at a depth of 30km, as an intraplate earthquake in the outer rise, featuring a mostly strike-slip mechanism expressing tensional stress parallel to the direction of convergence, with a moment of 5× 10 ²⁷ dyn cm. Two other large historical earthquakes south of Java (in 1937 and 1943) are also shown to be intraplate shocks, so that the Java subduction zone lacks large interplate thrust events for the entire era of instrumental seismicity. However, this does not violate the extrapolation to large sources of the frequency-moment characteristics of modern digital data (1976-2011), thus leaving open the possibility that the Java trench might entertain occasional, if rare, mega-thrust events.